The horse tested positive for banned local anaesthetic pramoxine during a competition in Wellington in the United States on January 21.

Johnson has already appealed to the Court of Arbitration for Sport after an FEI Tribunal studied the case and issued the ban.

The sanction has been backdated to begin on April 4, 2017, the date when she was provisionally suspended for three months.

She has also been fined CHF2,000 (£1,600/$2,070/€1,700) and must pay CHF3,000 (£2,300/$3,100/€2,600) in legal costs. 

A hearing heard that the horse's groom, Sergio Molinero, had bought ointment to treat Luke Skywalker's cuts.

This had been used before and was allowed under doping rules, but he bought the incorrect product at a Walmart store which caused the horse to test positive.

"I found the triple antibiotic on the shelves in the same spot it always is and pulled four tubes of it off the shelves," Molinero said, according to documents published by the FEI.

"I believed at the time I was buying the same triple antibiotic we always buy which is okay under the anti-doping rules. 

"I now realise after Paige was able to find my receipt for the purchase that I made a mistake and pulled the wrong tube off the shelf because it looked so much like the one we always use.

"I now see that I mistakenly bought triple antibiotic with pain relief, and the pain relief contains pramoxine."

The FEI ruled that Johnson did not take "every conceivable effort" to avoid that a banned substance entering the horse's system.

They ruled that she was negligent but that her fault was not significant, stating that Johnson "seems to generally take good care for the welfare of her horses, and generally has good anti-doping procedures in place".